My experience with orthodontists is that they are happy to put you on a payment plan if you can't afford to pay it all outright. Your initial consultation should only cost around $100 and they usually give you a quote for the work they'll do so you'll know exactly what you're up for in advance. I don't think orthodontic treatment is covered by the medicare dental benefit.

My nephew just saw an ortho this week for a similar problem. The dr was reluctant to start any treatment at his age (11yo), he wants to wait until he has lost all of his baby teeth. They may not want to wait if your dd has no adult teeth at all though, he is missing some but not all, he also has teeth growing in the roof of his mouth.